The present invention relates to developer compositions for electrophotograhic copying, and more particularly to a method of preparing an improved developer composition.
Various methods of electrophotographic development are knowns. One of the more common techniques is the magnetic brush development method. The magnetic brush development method utilizes magnetic means associated with a developing mixture of a magnetic carrier (e.g., particles or powder) carrying a number of smaller electrostatically adhering toner particles. The developer composition is maintained during the development cycle in a loose, brush-like orientation by a magnetic field surrounding, for example, a rotatable non-magnetic cylinder having a magnetic means fixedly mounted inside. The magnetic carrier is attracted to the cylinder by the described magnetic field, and the toner particles are held to the carrier by virtue of their opposite electrostatic polarity. Before and during development, the toner acquires an electrostatic charge of a sign opposite to that of the carrier due to triboelectric charging derived from their mutual frictional interaction. When this brush-like mass of magnetic carrier with adhering toner particles is drawn across the photoconductive surface bearing the electrostatic image, the toner particles are electrostatically attracted to an oppositely charged latent image and form a visible toner image corresponding to the electrostatic image.
Ferrite cores have been widely used as the magnetic carrier material. There are several critical parameters which are necessary and must be controlled to produce a suitable ferrite carrier core. First, the particulate shape, size, and size distribution affect the carrier coating and its ability to hold toner, thereby producing good copy quality. The volume resistivity of the ferrite must also be established in a range which will be compatible with the coating to control the developer conductivity. To utilize the developer in a magnetic brush development process, the ferrite core must have adequate saturation magnetization to control its carrier function. Further, the apparent density of the carrier materially affects the volume in the developer charge and provides proper flow characteristics. Two of the most important properties are the surface area and the surface morphology of the core; these have a direct impact on the coating characteristics of the core which in turn affect the copy quality.
To produce the optimum carrier characteristics, ferrites of both cubic and hexagonal structures have been used. It is preferred with the magnetic brush two-component developers to use the cubic spinel structure of the form MFe.sub.2 O.sub.4 or (MO).sub.x (Fe.sub.2 O.sub.3).sub.y where M is a metal selected from the group consisting of lithium, manganese, nickel, zinc, cadmium, copper, cobalt, iron, and magnesium such as described in U.S. Pat. No. 4,898,801 to Tachibana et al. Two of the most widely used systems are nickel zinc ferrite and copper zinc ferrite. Whereas these systems produce effective magnetic carriers, there is, however, growing concern regarding the environmental problems associated with the presence of various forms of nickel, zinc or copper such as found in the above carriers.
Other ferrites, are proposed, for example in U.S. Pat. No. 4,855,205 to Saha et al. Saha et al. proposes a two-phase cubic ferrite composition comprising 0.1 moles to 1.0 moles of a ferrite having the formula MFe.sub.2 O.sub.4 where M is at least one element which forms a spinel ferrite (e.g., nickel, cobalt, magnesium, manganese, copper, zinc, and iron) and about 2.5 moles of a magnetoplumbite having the formula R.sub.x P.sub.1-x Fe.sub.2 O.sub.19 where R is a rare earth and P is selected from the group consisting of strontium, barium, calcium and lead.
U.S. Pat. No. 5,004,665 to Ohtani et al. proposes a toner composition for electrophotographic copying comprising a resin, a colerant, and polymeric-magnetic coordination complex, poly-bis-(2,6-pyridinediylmethylidenenitrilohexamethylenenitrilomethyliden e) iron sulfate.
U.S. Pat. No. 3,590,000 to Palermiti proposes carrier particles comprised of various cores including steel, with a coating of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds.
U.S. Pat. No. 5,162,187 to Lyons et al. proposes a carrier composition comprised of a semiconductive ferrite core with a coating of a mixture of first and second polymers that are not in close proximity in the triboelectric series (e.g., polystyrene and tetrafluoroethylene). At column 5, lines 4 through 14, a specific semiconductive ferrite core comprising magnesium copper zinc available from the assignee of the present invention, Steward, Inc., is described.
Other ferrite carriers are proposed in U.S. Pat. Nos. 2,846,333 to Wilson; 3,929,657 to Jones; and 4,127,667 to Jones.
Despite the general availability of ferrite carriers and various methods of producing and using them, there continues to be a desire for improvements in carrier performance. For example, it is an object to provide higher volume resistivity and a wider range of saturation magnetization. It is also an object to eliminate or reduce the use of ferrites having forms of nickel, zinc, and copper. These metals have come under increased scrutiny by regulatory agencies, and materials including these metals often require special, and typically expensive, disposal methods. It is a further object to produce a suitable ferrite carrier inexpensively and efficiently.